This invention relates generally to electronic phase shifters and, more particularly to voltage tunable phase shifters for use at microwave and millimeter wave frequencies that operate at room temperature.
Tunable phase shifters using ferroelectric materials are disclosed in U.S. Pat. Nos. 5,307,033, 5,032,805, and 5,561,407. These phase shifters include ferroelectric substrate as the phase modulating elements. The permittivity of the ferroelectric substrate can be changed by varying the strength of an electric field applied to the substrate. Tuning of the permittivity of the substrate results in phase shifting when an RF signal lasses through the phase shifter.
One known type of phase shifter is the microstrip line phase shifter. Examples of microstrip line phase shifters utilizing tunable dielectric materials are shown in U.S. Pat. Nos. 5,212,463; 5,451,567 and 5,479,139. These patents disclose microstrip lines loaded with a voltage tunable ferroelectric material to change the velocity of propagation of a guided electromagnetic wave.
Tunable ferroelectric materials are materials whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called xe2x80x9cferroelectricxe2x80x9d because they exhibit spontaneous polarization at temperatures below the Curie temperature. Tunable ferroelectric materials including barium-strontium titanate (BST) or BST composites have been the subject of several patents.
Dielectric materials including barium strontium titanate are disclosed in U.S. Pat. No. 5,312,790 to Sengupta, et al. Entitled xe2x80x9cCeramic Ferroelectric Materialxe2x80x9d; U.S. Pat. No. 5,427,988 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-MgOxe2x80x9d; U.S. Pat. No. 5,486,491 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-ZrO2xe2x80x9d; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-Magnesium Based Compoundxe2x80x9d; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled xe2x80x9cMultilayered Ferroelectric Composite Waveguidxe2x80x9d; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled xe2x80x9cThin Film Ferroelectric Composites and Method of Makingxe2x80x9d; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled xe2x80x9cMethod of Making Thin Film Compositesxe2x80x9d; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled xe2x80x9cElectrically Graded Multilayer Ferroelectric Compositesxe2x80x9d; and U.S. Pat. No. 5,635,433 to Sengupta, entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-ZnOxe2x80x9d. These patents are hereby incorporated by reference. A copending, commonly assigned U.S. patent application Ser. No. 09/594,837 titled xe2x80x9cElectronically Tunable Ceramic Materials Including Tunable Dielectric And Metal Silicate Phasesxe2x80x9d, by Sengupta, filed Jun. 15, 2000, and issued Jun. 11, 2002 as U.S. Pat. 6,404,614 discloses additional tunable dielectric materials and is also incorporated by reference. The materials shown in these patents, especially BSTO-MgO composites, show low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage.
Adjustable phase shifters are used in many electronic applications, such as for beam steering in phased array antennas. A phased array refers to an antenna configuration composed of a large number of elements that emit phased signals to form a radio beam. The radio signal can be electronically steered by the active manipulation of the relative phasing of the individual antenna elements. Phase shifters play a key role in operation of phased array antennas. The electronic beam steering concept applies to antennas used with both a transmitter and a receiver. Phased array antennas are advantageous in comparison to their mechanical counterparts with respect to speed, accuracy, and reliability. The replacement of gimbals in mechanically scanned antennas with electronic phase shifters in electronically scanned antennas increases the survivability of antennas used in defense systems through more rapid and accurate target identification. Complex tracking exercises can also be maneuvered rapidly and accurately with a phased array antenna system.
U.S. Pat. No. 5,617,103 discloses a ferroelectric phase shifting antenna array that utilizes ferroelectric phase shifting components. The antennas disclosed in that patent utilize a structure in which a ferroelectric phase shifter is integrated on a single substrate with plural patch antennas. Additional examples of phased array antennas that employ electronic phase shifters can be found in U.S. Pat. Nos. 5,079,557; 5,218,358; 5,557,286; 5,589,845; 5,617,103; 5,917,455; and 5,940,030.
U.S. Pat. Nos. 5,472,935 and 6,078,827 disclose coplanar waveguides in which conductors of high temperature superconducting material are mounted on a tunable dielectric material. The use of such devices requires cooling to a relatively low temperature. In addition, U.S. Pat. Nos. 5,472,935 and 6,078,827 teach the use of tunable films of SrTiO3, or (Ba, Sr)TiO3 with high a ratio of Sr. ST and BST have high dielectric constants, which results in low characteristics impendence. This makes it necessary to transform the low impendence phase shifters to the commonly used 50 ohm impedance.
Low cost phase shifters that can operate at room temperature could significantly improve performance and reduce the cost of phased array antennas. This could play an important role in helping to transform this advanced technology from recent military dominated applications to commercial applications.
There is a need for electrically tunable phase shifters that can operate at room temperatures and at K and Ka band frequencies (18 GHz to 27 GHz and 27 GHz to 40 GHz, respectively), while maintaining high Q factors and have characteristic impedances that are compatible with existing circuits.
Certain embodiments of the invention provide a phase shifter including a substrate, a tunable dielectric film having a dielectric constant between 70 to 600, a tuning range of 20% to 60%, and a loss tangent between 0.008 to 0.03 at K and Ka bands, the tunable dielectric film being positioned on a surface of the substrate, a coplanar waveguide positioned on a top surface of the tunable dielectric film opposite the substrate, an input for coupling a radio frequency signal to the coplanar waveguide, an output for receiving the radio frequency signal from the coplanar waveguide, and a connection for applying a control voltage to the tunable dielectric film.
The invention also encompasses a reflective termination coplanar waveguide phase shifter including a substrate, a tunable dielectric film having a dielectric constant between 70 to 600, a tuning range of 20 to 60%, and a loss tangent between 0.008 to 0.03 at K and Ka bands, the tunable dielectric film being positioned on a surface of the substrate, first and second open ended coplanar waveguide lines positioned on a surface of the tunable dielectric film opposite the substrate, a microstrip line for coupling a radio frequency signal to and from the first and second coplanar waveguide lines, and a connection for applying a control voltage to the tunable dielectric film.
The conductors forming the coplanar waveguide operate at room temperature. The coplanar phase shifters of the present invention can be used in phased array antennas at wide frequency ranges. The devices herein are unique in design and exhibit low insertion loss even at frequencies in the K and Ka bands. The devices utilize low loss tunable film dielectric elements.